solid angle of rectangular pyramid

We calculate the apical solid angle of a rectangular pyramid, as an example of using the http://planetmath.org/node/7266formula of van Oosterom and Strackee for determining the solid angle Ω subtended at the origin by a triangle:

tan⁡Ω2=r→1×r→2⋅r→3(r→1⋅r→2)⁢r3+(r→2⋅r→3)⁢r1+(r→3⋅r→1)⁢r2+r1⁢r2⁢r3

(1)

Here, r→1, r→2, r→3 are the position vectors of the vertices of the triangle and
r1,r2,r3 their .

Let the apex of the pyramid be in the origin and the vertices of the base rectangle be

(±a,±b,h)

where a, b and h are positive numbers. We take the half-triangle of the base determined by the three vertices